The present invention relates to semiconductor laser devices, and in particular, to a semiconductor laser chip mounting structure and fabricating method and an optical pickup employing the device.
Prior art semiconductor laser devices are shown in FIG. 6 through FIG. 9, and the die bonding process of one prior art semiconductor laser device fabricating method is shown in FIGS. 10A and 10B.
In the prior art semiconductor laser device of FIG. 6, a semiconductor laser chip 50 is placed in a specified position of a header portion 51a located at an end of a stem 51 via a metal brazing material (solder or the like) 52. The semiconductor laser chip 50 is bonded by the metal brazing material 52 in the place where the chip is to be die-bonded. Therefore, in the bonding stage, the semiconductor laser chip 50 is required to be fixed by a bonding collet (not shown) or the like so that the chip does not move. In FIG. 6, an emission light optical axis 55 of the semiconductor laser device is an axis that connects a main radiation side light emission point 53 with a monitor side light emission point 54.
There are the preceding references of, for example, Japanese Patent Laid-Open Publication No. SHO 63-138794 and Japanese Patent Laid-Open Publication No. HEI 5-291696, each of which employs a metal brazing material (gold-tin alloy solder or the like) and pays attention to the size of the semiconductor laser chip and the size of a protruding portion (or a header portion) of a mount. These devices are now described with reference to FIG. 7 and FIG. 8.
Referring to FIG. 7, a prior art semiconductor laser device 60 is obtained by forming a mount 63 by mesa-etching silicon, die bonding a semiconductor laser element 61 whose active layer 62 faces the mount side to a protruding portion of the mount 63 by means of a gold-tin alloy solder 64 and bonding a gold wire 66 to a semiconductor laser element 61. A heat radiating plate 65 is provided underneath the mount 63.
As shown in FIG. 7, the semiconductor laser element 61 and the mount 63 are not put in contact with each other in the peripheral portion of the semiconductor laser element 61. Therefore, the gold-tin alloy solder 64 that oozes out of the upper surface of the protruding portion of the mount 63 in the die bonding stage of the semiconductor laser element 61 stays around the protruding portion of the mount 63 and does not rise on the side surfaces of the semiconductor laser element 61.
FIG. 8 shows a sectional view of another prior art semiconductor laser device 70. The semiconductor laser device 70 is provided with a laser chip 71 and a heat sink 72. The heat sink 72 is provided with a protruding portion 80 that has a trapezoidal cross-section shape, and the upper surface of the protruding portion 80 is slightly smaller than the lower surface of the laser chip 71 and has a flat mounting surface 72a. The laser chip 71 is mounted on the upper surface of the protruding portion 80, i.e., the mounting surface 72a via a brazing material 73.
Fabrication of this semiconductor laser device 70 includes the steps of coating a low-melting-point brazing material (Indium (In), for example) 73 on the upper surface of the protruding portion 80, performing melting with heat (temperature of 300xc2x0 C.) and cooling and mounting the laser chip 71 on the protruding portion 80.
As described above, the laser chip 71 is directly die-bonded (direct bonding system) to the protruding portion 80 of the heat sink 72 by means of the brazing material 73 with interposition of no sub-mount. Therefore, the semiconductor laser device can be fabricated at low cost. The brazing material 73 pushed out of the mounting surface 72a of the heat sink 72 by the laser chip 71 creeps on the surfaces of the protruding portion 80 when melted with heat and does not rise on the side surfaces of the laser chip 71. Therefore, even when a cap layer 75 is made thin to put a light-emitting section 74 close to the heat sink 72, a laser beam L can be prevented from being diffusively reflected or partially hampered by the brazing material 73, and the radiation characteristic can be improved.
In recent years, there is a demand for improving the productivity of the semiconductor laser device fabricating method through value engineering (VE) or the like by an increase in mounting efficiency, a reduction in the number of processes and mechanization. From this standpoint, the conventional semiconductor laser devices have had the problems that much time is necessary for the heating and cooling cycles of the metal brazing material (gold-tin alloy solder or a low-melting-point brazing material In) and that the material cost of the metal brazing material is high.
As a measure for improvement, there is the semiconductor laser device shown in FIG. 9. In FIG. 9 are shown a semiconductor laser chip 50, a stem 51, a header portion 51a of the stem, a main radiation side light emission point 53, a monitor side light emission point 54 and a semiconductor laser device emission light optical axis 55 that connects the main radiation side light emission point with the monitor side light emission point. In the semiconductor laser device shown in FIG. 9, the semiconductor laser chip 50 is die-bonded by means of a conductive die bonding paste 56 employed in place of the metal brazing material 52. If the conductive die bonding paste 56 is employed, then the material cost is inexpensive, and the heating and curing of the paste can be performed after the die bonding. Therefore, if the die bonding paste 56 is employed, then there is no need for heating and cooling the semiconductor laser device in the bonding place or by means of a bonding apparatus. This enables the reduction in time of the die bonding process and the reduction in the occupation time of the bonding place (or the bonding apparatus). The die-bonded semiconductor laser device is moved to another place and subjected to the heating and cooling processes.
FIGS. 10A and 10B show the die bonding process of the aforementioned prior art semiconductor laser device fabricating method. In FIG. 10A, a specified trace quantity of conductive die bonding paste 56 is ejected from a needle tip 57 of a syringe needle of a dispenser, and the needle tip 57 of the syringe needle is moved in a downward direction 58A. The conductive die bonding paste 56 is coated in a specified position of the header portion 51a of the stem 51 by the descent of the syringe needle, and thereafter, the needle tip 57 of the syringe needle is moved in an upward direction 58B to put the syringe needle apart as shown in FIG. 10B. Subsequently, the semiconductor laser chip 50 is placed on the coated conductive die bonding paste 56.
The semiconductor laser chip 50 has a size of about 0.2 mm square, and the light emission point is located at a height of about 0.05 mm from the lower surface of the semiconductor laser chip 50. On the other hand, the needle tip 57 of the syringe needle has a diameter of about 0.3 mm. The needle tip 57 of the syringe needle should preferably be small. However, in order to reliably coat a specified quantity of conductive die bonding paste 56, the needle tip size cannot be set smaller than a diameter of about 0.3 mm.
Therefore, due to the fact that the syringe needle tip 57 has the size of a diameter of about 0.3 mm and the fact that the semiconductor laser chip 50 has the size of about 0.2 mm square, the conductive die bonding paste 56 is coated in an area broader than that of the semiconductor laser chip 50.
However, in the aforementioned prior art semiconductor laser device, the conductive die bonding paste 56 discharged from the lower surface of the semiconductor laser chip 50 has a thickness of up to about 0.05 mm in relation to the viscosity of the conductive die bonding paste. On the other hand, the light emission point of the semiconductor laser chip 50 exists at a height of about 0.05 mm from the lower surface of the semiconductor laser chip 50. Therefore, when the semiconductor laser chip 50 is die-bonded by means of the conductive die bonding paste 56, the conductive die bonding paste 56 rises on the end surfaces and the side surfaces of the semiconductor laser chip 50 as shown in FIG. 9, and this tends to cause the problem that the main radiation side light emission point 53 and the monitor side light emission point 54 are disadvantageously concealed.
Furthermore, there have lately been developed applications of optical pickups employing a semiconductor laser device, such as optical disks, and high output power lasers having an optical output of not smaller than 50 mW have been increasingly used for the optical pickups of erasable information. However, according to an optical pickup and, in particular, an optical pickup that employs a 3-beam system, return light of side beams reflecting on the chip surface, stem surface and the like of the optical pickup, exerts a bad influence. Accordingly, there has been a growing demand for eliminating the bad influence of the return light.
Accordingly, the object of the present invention is to provide a semiconductor laser device in which the conductive die bonding paste conceals neither a main radiation side light emission point nor a monitor side light emission point and the return light of the optical pickup exerts no bad influence as well as the fabricating method of the device and an optical pickup employing the device.
In order to solve the aforementioned object, the present invention provides a semiconductor laser device comprising a semiconductor laser chip placed on a stem in such a manner that an end surface of the semiconductor laser chip on which a light emission point on main radiation side is located protrudes from an edge of a header portion of the stem or from an edge of a header portion of a sub-mount provided on the stem so as to conceal no light emission points of the semiconductor, and wherein a conductive die bonding paste is employed as an adhesive for die bonding of the semiconductor laser chip.
Accordingly, there can be obtained a high-reliability high-productivity semiconductor laser device, in which the adhesive does not rise on the end surface and the side surface of the semiconductor laser chip in the die bonding stage and the trouble of concealing the main radiation side light emission point and the monitor side light emission point does not occur.
In one embodiment of the present invention, the conductive die bonding paste is applied on a rear surface of the semiconductor laser chip that is placed in such a manner as to protrude from the edge of the header portion of the stem or from the edge of the header portion of the sub-mount.
Accordingly, by providing the adhesive on the rear surface of the semiconductor laser chip in the die bonding stage, this conductive die bonding paste rigidly supports and protects the semiconductor laser chip so provided as to protrude from the edge of the header portion of the stem or the sub-mount. Therefore, a highly reliable semiconductor laser device can be obtained.
In one embodiment of the present invention, a chamfered portion or a rounded corner portion is formed at the edge of the header portion of the stem or the sub-mount provided on the stem.
Accord to the embodiment, the chamfered portion or a rounded corner portion is formed, the conductive die bonding paste does not rise on the end surface and the side surface of the semiconductor laser chip and the trouble of concealing the main radiation side light emission point and the monitor side light emission point does not occur. Furthermore, the conductive die bonding paste drooping on the chamfered portion or the like is hard to reflect light. This prevents the semiconductor laser device from being defective due to the reflection of return light and enables the obtainment of a highly reliable semiconductor laser device having a tolerance to the return light.
In one embodiment of the present invention, the light emission point of the semiconductor laser chip is located about 0.03 millimeter or more in height than a die bonding surface of the semiconductor laser chip.
Accordingly, by highly setting the height of the light emission point of the semiconductor laser chip to 0.03 millimeter or more, there can be obtained a high-reliability high-productivity semiconductor laser device, in which the conductive die bonding paste does not rise on the end surface and the side surface of the semiconductor laser chip and the trouble of concealing the main radiation side light emission point and the monitor side light emission point does not occur.
The present invention provides an optical pickup comprising the semiconductor laser device stated above, a diffraction grating and a photodetector.
Accordingly, by employing the semiconductor laser device of the present invention, the optical pickup that receives less influence from the return light of the semiconductor laser device can be obtained.
The present invention provides a method for fabricating the semiconductor laser device as stated above, comprising the step of arranging a syringe needle in such a position that a syringe needle tip partially protrudes from the edge of the header portion of the stem or from the edge of the header portion of the sub-mount provided on the stem when the conductive die bonding paste is ejected from the syringe needle tip of a dispenser so as to be coated.
According to the method for fabricating the semiconductor laser device, the adhesive does not rise on the end surface and the side surface of the semiconductor laser chip in the die bonding stage and the trouble of concealing the main radiation side light emission point and the monitor side light emission point does not occur. Therefore, there can be obtained a highly reliable semiconductor laser device.
Furthermore, by employing the conductive die bonding paste as the die bonding adhesive of the semiconductor laser chip, the material cost is less expensive than that of the metal brazing material or the like. In addition, the heating and curing of the conductive die bonding paste is allowed to be performed after the die bonding, meaning that neither heating nor cooling in the place or apparatus of die bonding is required. This enables the reduction in time of the die bonding process and the reduction in the occupation time of the bonding place (apparatus). As a result, an inexpensive high-reliability high-productivity semiconductor laser device and a fabricating method thereof can be obtained.